4 November 2010: Five British soldiers killed by Afghan colleague in Helmand

"While this is a serious incident, the actions of this individual do not reflect the overall actions of our Afghan partners," said US Marine Corps Maj Gen James Laster, International Security Assistance Force Joint Command Deputy Chief of Staff Joint Operations.

"We remain committed to our partners and to our mission here."

Nato's exit strategy for Afghanistan involves progressively handing over to the local security forces.

Seven provinces and cities were named in the first tranche of areas to be transferred to local control in July.

There are now more than 260,000 Afghan security personnel, of whom more than 160,000 were trained over the past year.

The US is investing $11bn (£6.82bn) a year in the training mission.

But with so many new recruits being taken on, there are questions over the vetting process, the extent to which the Taliban may have infiltrated those forces, and their loyalty and reliability, correspondents say.